1. Field of the Invention
The present invention relates to a method for regenerating a metal bath contaminated by mixed crystals, in particular a tin bath of a hot-dip coating process, in which the metal bath is first heated above its liquidus temperature and then the mixed crystals coming down during cooling are mechanically removed. The invention also relates to an arrangement for carrying out such a method.
2. Description of Related Art
Metal objects are mostly provided with metal coatings for reasons of corrosion and wear protection, or to attain greater surface hardness. Improvement of the sliding properties or achieving a decorative look can also be the aim of such a coating. A pure metallic surface coating is demanded especially with copper materials in many of its uses and industrial applications, for example, in producing electrical plug connectors.
Use of the metal bath method is widespread, especially for coating metal bands, for example in hot-dip tinning or galvanizing. In these processes, a metal tape and fluxing salts are guided through a metal bath consisting of the coating metal. As a result of diffusion processes between the metal atoms of the fluid coating metal and the atoms of the base metal alloy layers are formed. When the metal tape is pulled out of the metal bath, a layer of pure coating metal is found on its surface. The thickness of the coating""s layer is set either by mechanical stripping off or by blowing it off, using air or protective gas.
During the course of the coating process, a metal bath is contaminated by chemical reaction. The cause for this is especially that metal or alloy components of the dipped objects are dissolved in the metal bath. Cracking of unused flux also causes contamination. This chemical change of the metal bath deteriorates the coating results, mainly by changing the consistency of the metal bath, and thereby its wetting performance. The molten baths subsequently become unusable with increasing working life.
By increasing the operating temperature of the metal bath the working life can be prolonged. Though, this measure leads to an increase in the solution effect, with the result that the contamination of the metal bath increases faster and more strongly.
When the maximum permissible concentration of undesired components has been reached, the coating tank is taken out of the line and cleaned. This happens intermittently. To do this, the metal bath is first heated to a temperature above its liquidus temperature. Above this temperature all alloy components go into solution. The contaminants in the metal bath, mostly mixed crystals of combinations of the base metal and the coating metal, also go back into solution. After that, stainless steel strainers are pressed into the metal bath. Such strainers usually have a mesh size of 5 mm. The metal bath is then cooled convectively, until the temperature has sunk to the level where the mixed crystals come down. Now the strainers are pulled out of the metal bath, and the mixed crystals are thereby mechanically removed.
This procedure is repeated several times, until the desired regeneration of the metal bath has taken place, and chemical analysis confirms the desired composition of the metal bath. The contaminants form the metal bath remain in the strainers, and are subsequently burned out thermally and returned for further use. Alternatively, the mixed crystals can be spooned out of the tank sump with the aid of a perforated ladle.
Frequently the separation process has to be repeated up to ten times before a desired composition of the metal bath is reached. As a result, this method for regenerating the metal bath is time and labor intensive.
It is an object of the invention to provide a method and an arrangement which make possible efficient regeneration of a metal bath contaminated by mixed crystals, when using efficient and economical working procedures. This and other objects of the invention are achieved by a method for regenerating a metal bath contaminated by mixed crystals, particularly a tin bath in a hot-dip coating process, in which the metal bath is first of all heated above its liquidus temperature, and then the mixed crystals coming down during cooling are mechanically removed, wherein the metal bath, after being heated to at least its liquidus temperature is conducted over a cooling unit and, during the flow-through, is guided through at least one filter. An arrangement for carrying out this method includes a pump for conveying metal bath from a tank to the cooling unit and to the filter downstream from it.